Lecture 6: Collections ppt

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Collections
Lecture 6:
2
Concept

A collection is a data structure – actually, an object – to hold other
objects, which let you store and organize objects in useful ways for
efficient access

Check out the java.util package! Lots of interfaces and classes
providing a general collection framework. Programmers may also
provide implementations specific to their own requirements

Overview of the interfaces and concrete classes in the collection
framework
Collection
Set
SortedSet
List
HashSet
ArrayList
LinkedList
TreeSet
Map
SortedMap
WeakHashMap
HashMap
TreeMap
ListIterator

Iterator
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Root interface – Collection (1)

Methods working with an individual collection

public int size()

public boolean isEmpty()

public boolean contains(Object elem)

public boolean add(Object elem)

Depends on whether the collection allows duplicates

public boolean remove(Object elem)

public boolean equals(Object o)

public int hashCode()

public Iterator iterator()

public Object[] toArray()

Returns a new array containing references to all the elements of the
collection

public Object[] toArray(Object[] dest)


What is returned depends on whether the elements in the collection fit in
dest

If the type of dest is not compatible with the types of all elements in the
collection, an exception is thrown
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Root interface – Collection (2)

Primary methods operating in bulk from another collection

public boolean containsAll(Collection coll)

public boolean addAll(Collection coll)

Returns true if any addition succeeds

public boolean removeAll(Collection coll)

Returns true if any removal succeeds

public boolean retainAll(Collection coll)

Removes from the collection all elements that are not elements of coll

public void clear()

Remove all elements from this collection

The SDK does NOT provide any direct implementations of the

Collection interface

Most of the actual collection types implement this interface, usually by
implementing an extended interface such as Set or List

This interface is typically used to pass collections around and
manipulate them where maximum generality is desired.
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Iteration - Iterator

The Collection interface defines an iterator method to
return an object implementing the Iterator interface.

It can access the elements in a collection without exposing its internal
structure.

There are NO guarantees concerning the order in which the elements
are returned

Three defined methods in Iterator interface

public boolean hasNext() – returns true if the iteration has more
elements

public Object next() – returns the next element in the iteration

An exception will be thrown if there is no next element

What’s returned is an Object object. You may need special casting!


public void remove() – remove from the collection the element
last returned by the iteration

can be called only once per call of next, otherwise an exception is
thrown
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classical routine of using iterator:
public void removeLongStrings (Collection coll, int
maxLen) {
Iterator it = coll.iterator();
while ( it.hasNext() ) {
String str = (String)it.next();
if (str.length() > maxLen)
it.remove()
}
}
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Iteration - ListIterator

ListerIterator interface extends Iterator interface. It adds
methods to manipulate an ordered List object during iteration

Methods

public boolean hasNext()/ public boolean hasPrevious()

public Object next()/ public Object previous()

public Object nextIndex()/ public Object previousIndex()


When it’s at the end of the list, nextIndex() will return list.size()

When it’s at the beginning of the list, previousIndex() will return -1

public void remove() – remove the element last returned by next() or
previous()

public void add(Object o)– insert the object o into the list in front of the
next element that would be returned by next(), or at the end if no next
element exists

public void set(Object o) – set the element last returned by next() or
previous() with o
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
They do NOT provide the snapshot guarantee – if the content of the
collection is modified when the iterator is in use, it can affect the
values returned by the methods
import java.util.*;
public class IteratorTest {
public static void main (String args[]) {
ArrayList a = new ArrayList();
a.add("1");
a.add("2");
a.add("3");
Iterator it = a.iterator();
while(it.hasNext()) {
String s = (String)(it.next());
if(s.equals(“1")) {
a.set(2,“changed");

}
System.out.println(s);
}
}
}
Potential problem of Iterator/ListIterator
Output?
1
2
changed
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Potential problem of Iterator/ListIterator
(cont.)

A snapshot will return the elements as they were when the
Iterator/ListIterator object was created, which is
unchangeable in the future

If you really need a snapshot, you can make a simple copy of
the collection

Many of the iterators defined in the java.util package are in
the type of fail-fast iterators

They detect when a collection has been modified

When a modification is detected, other than risk performing an
action whose behavior may be unsafe, they fail quickly and cleanly
by throwing an exception –
ConcurrentModificationException

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import java.util.*;
public class IteratorTest2 {
public static void main (String args[]) {
ArrayList a = new ArrayList();
a.add(“1”);
a.add(“2”);
a.add(“3”);
Iterator it = a.iterator();

a.add(“4”);
while(it.hasNext()) {
String s = (String)(it.next());
System.out.println(s);
}
}
}
%> javac IteratorTest2.java
%> java IteratorTest2
Exception in thread “main” java.util.ConcurrentModificationException
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List

A List is an ordered Collection which allows duplicate
elements. Its element indices range from 0 to
(list.size()-1)

It adds several methods for an ordered collection

The interface List is implemented by two classes

1. ArrayList: a resizable-array implementation of the List
interface

Adding or removing elements at the end, or getting an element at a specific
position is simple – O(1)

Adding or removing element from the middle is more expensive – O(n-i)

Can be efficiently scanned by using the indices without creating an
Iterator object, so it’s good for a list which will be scanned frequently
2. LinkedList: a doubly-linked list

Getting an element at position i is more expensive – O(i)

A good base for lists where most of the actions are not at the end

An example of using LinkedList (output)
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Set and SortedSet

The Set interface provides a more specific contract for its
methods, but adding no new methods of its own. A Set is a
Collection that contains UNIQUE elements.

The SortedSet extends Set to specify an additional contract –
iterators on such a set will always return the elements in a
specified order

By default it will be the elements’ natural order which is determined by
the implementation of Comparable interface


You can specify a Comparator object to order the elements instead
of the natural order

There are two implementations of Set in the collection framework

HashSet – a Set implemented using a hashtable

TreeSet – a SortedSet implemented in a balanced tree structure

An example of using a HashSet
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Map and SortedMap

The Map interface does not extend Collection interface because
a Map contains key-value pairs, not only keys. Duplicate keys are
not allowed in a Map. It’s implemented by classes HashMap and
TreeMap.

There are methods to view the map using collections. For example:
public Set keySet() and public Collection values().

The collections returned by these methods are backed by the Map, so
removing an element from one these collections removes the
corresponding key/value pair from the map

You cannot add elements to these collections

If you iterate through the key or value sets, they may return values from
their respective sets in any order


Interface SortedMap extends Map and maintains its keys in sorted
order. Class TreeMap implements SortedMap.

An example using HashMap
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Synchronized wrappers and the Collections class (1)

The Collections class contains static utility methods which can
be roughly classified into two groups: those provide wrapped
collections and those don’t.

All the collection implementations provided in java.util we’ve
seen so far are unsynchronized

concurrent access to a Collection by multiple threads could cause
indeterminate results or fatal errors.

you can use synchronization wrappers for those collections that might
be accessed by multiple threads to prevent potential threading
problems.

Methods in the Collections class to get a synchronized wrapper

Collection synchronizedCollection(Collection c)

Set synchronizedSet(Set s)

SortedSet synchronizedSortedSet(SortedSet s)


List synchronizedList(List l)

Map synchronizedMap(Map m)

SortedMap synchronizedSortedMap(SortedMap m)
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Synchronized wrappers and the Collections class (2)

The above methods return wrappers whose methods are fully
synchronized, and so are safe to use from multiple threads
Example
Map unSyncMap = new HashMap();
Map syncMap = Collections.synchronizedMap(unSyncMap);

synchMap has all relevant methods synchronized, passing all calls
through to the wrapped map (unSynchMap)

there is actually only one map, but with two different views. So
modifications on either map is visible to the other

the wrapper synchronizes on itself, so you can use syncMap to
synchronize access, and then use unsyncMap safely inside such code
synchronized (syncMap) {
for (int i=0; i< keys.length; i++)
unSyncMap.put ( keys[i], values[i] );
}
HashMap
synchronized
wrapper
syncMap

unSyncMap
elements
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Unmodifiable wrappers and the Collections class (1)

The Collections class contains a set of methods that return
unmodifiable wrappers for collections: attempts to modify the returned
set, whether direct or via its iterator, result in an
UnsupportedOperationException

The contents of an unmodifiable wrapper can change, but can only
through the original collection, not through the wrapper itself

Six methods to return unmodifable wrappers:

Collection unmodifiableCollection(Collection c)

Set unmodifiableSet(Set s)

SortedSet unmodifiableSortedSet(SortedSet s)

List unmodifiableList(List l)

Map unmodifiableMap(Map m)

SortedMap unmodifiableSortedMap(SortedMap m)
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Example
Original: it’s dangerous that the array’s content can be changed
public String suits[]= {

“Hearts”, “Clubs”, “Diamonds”, “Spades” };
Using the unmodifiable wrapper to prevent the danger:
private String suitsNames[] = {
“Hearts”, “Clubs”, “Diamonds”, “Spades” };
public final List suits =
Collections.unmodifiableList(Arrays.asList(suitNames)
;

The unmodifiable wrapper offers read-only access to
others, while the read-write access is still available to the
code itself by retaining a reference to the wrapped
collection (the original collection)
Unmodifiable wrappers and the Collections class (2)
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Abstract implementations

The collection framework provides a set of abstract implementations
for you to design your own implementation of relevant collection
interfaces to satisfy your particular needs

The set of abstract classes:

AbstractCollection

AbstractSet

AbstractList

AbstractSequentialList


AbstractMap
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The legacy collection types

The package java.util contains some other legacy collections
than those we just learned. They are still in wide use in existing
code and will continue to be used until programmers shift over to
the new types

The set of legacy collections

Enumeration – analogous to Iterator

Vector – analogous to ArrayList

Stack – a subclass of Vector

Dictionary – analogous to Map interface

Hashtable – analogous to HashMap

Properties – a subclass of HashTable